This invention relates to a proportional type electromagnetic valve which is provided in an air-fuel mixture supply device adapted to supply fuel and air to an internal combustion engine and which controls the amount of fuel or air.
Generally, this type of electromagnetic valve is required to be able to control the flow rate in substantially proportional relationship to the input current.
A conventional proportional type electromagnetic valve has integrally formed electromagnetic driving section and flow rate control valve, and a bias spring for urging the flow rate control valve. The bias spring urges the flow rate control valve to cope with electromagnetic force which acts on the control valve when the electromagnetic driving section is fed with an electric current, so that the stroke of the control valve is determined depending on the balance between the bias spring force and the electromagnetic force.
As described above, in the proportional type electromagnetic valve it is required that the flow rate be controllable over a wide range in proportion to the electric current fed to the electromagnetic driving section.
The proportional type electromagnetic valve of the prior art was, however, difficult to achieve such a characteristic as above. More particularly, when a coil of the electromagnetic driving section is fed with a current, electromagnetic force proportional to the current acts on the control valve and the stroke of the valve is determined corresponding to a position at which the electromagnetic force and the force of the bias spring are balanced. Actually, however, such factors as nonlinearity of the electromagnetic force, frictional resistance of a movable portion, and non-lineality of the bias spring force degrade the proportional relation between the current and the stroke.
This requires a suitable selection of the spring constant of the bias spring. If the bias spring is soft and has a small spring constant, the controlled flow rate, on one hand, will rapidly be increased and saturated within a range of a small current and the flow rate control valve, on the other hand, cannot fully close even when the current is rendered off. Moreover, an increase in the width of hysteresis is accompanied by variations of the current. Conversely, if the bias spring is stiff and has a large spring constant, the set load (a load with which the spring begins to deform) will inevitably increase. Consequently, it is not before the current reaches a relatively large predetermined value that the flow rate control valve begins to operate, and even with an increased current the movement of the flow rate control valve is suppressed to be small.
Under these conditions, if the current fed to the coil is excessively increased so as to extend the movement of the flow rate control valve for its full open, the power transistor of a driving circuit will be overheated.
As will be seen from the above, it is impossible to properly control the movement of the flow rate control valve with a single bias spring.
An object of the present invention is to provide a proportional type electromagnetic valve being suitable for producing the flow rate which is in exact proportion to the amount of input current. In the proportional type electromagnetic valve according to the invention, there is provided, in addition to a bias spring, a second spring which urges a flow rate control valve in the opposite direction to the bias spring to open the flow rate control valve, whereby the flow rate can be controlled in exact proportion to the amount of the current fed to an electromagnetic driving section.